The Sargassum subgenera Bactrophycus and Arthrophycus were considered to be geographically restricted to the northern and southern hemispheres, respectively. Phylogenetic analyses of 19 Sargassum subgenus Bactrophycus species and eight Sargassum subgenus Arthrophycus species, based on a concatenated dataset of the loci ITS-2, cox3 and the rbcL-S spacer, showed that they formed a single clade, with Arthrophycus species nested within Bactrophycus section Halochloa. We merged the two subgenera as subgenus Bactrophycus and transferred "Arthrophycus" species to Sargassum section Halochloa. The genus now includes only the two subgenera, Sargassum and Bactrophycus, and both were found at temperate and subtropical latitudes; only subgenus Sargassum occurred at low latitudes near the equator, whereas subgenus Bactrophycus had an antitropical, disjunct distribution.